(1) Field of the Invention
The present invention relates to a system and a method for controlling vehicle speed applicable to automatic transmission equipped vehicles in which output of an engine having a turbo charger is transmitted to a driven wheel via an automatic transmission.
The present invention particularly relates to a system and a method for controlling vehicle speed in which gear shifting control is carried out using switching pressure generated by means of on-and-off action of a plurality of solenoid valves.
(2) Background of the Art
In general, automatic transmissions mounted in vehicles include a combination of coupled toothed gear components including a plurality of pairs of planetary gears so that several stages of gear range changes may be carried out by hydraulically operated frictional elements.
A Japanese Patent Application First publication No. Tokkai Showa 62-62047 published on Mar. 18, 1987 exemplifies a shock reduced automatic transmission in which a working fluid pressure for engaging and releasing each frictional element is controlled by means of two shift valves. The change in the shift valves is carried out by means of a change pressure generated by on-and-off action of a solenoid valve, a so called shift solenoid installed corresponding to the shift valves.
An system for carrying out shift valve switching using solenoid valves has been applied to an RE4R01A type automatic transmission, described in "NISSAN FULL RANGE ELECTRONIC CONTROL AUTOMATIC TRANSMISSION", a service manual published in 1987.
On and off control of the shift solenoids can finely control the change shift valve pressure. For example, at a transient time of gear shift change in which the gear position is shifted from the first range to the second range, timing control is carried out such that one of the shift valves is changed from on to off within a short period of time, a rise in the engaged pressure of the frictional elements is gradually carried out. Thus, a reduction of the gear range shock can be achieved.
In addition, to reduce shock generated during a transient period of gear change, the engine output is reduced so that speed control of the vehicle is desirably carried out. This is exemplified by a Japanese Patent Application First publication Showa 63-71530 published on Mar. 31, 1983.
To reduce engine output, the ignition timing can normally be retarded as described in the Japanese Patent Application First Publication No. Showa 63-71530.
However, in previously proposed running speed controlling systems and methods, when internal cylinder pressure is increased with the ignition timing being retarded at the transient time of gear change, mis-firing occurs and engine operation becomes erratic. Conversely, if cylinder pressure is reduced, vibration at the time of gear change becomes great. Even though required secondary voltage may be reduced, an ignition device for igniting a mixture fuel supplied to the engine is large and costly.